I used to be very familiar with the standard trigger. I had taken mine apart... lost parts... repeat.

The double-trouble in the link looks like it has all the same parts as the standard trigger, but with an additional 2nd sear, linked at the opposite side of the trigger pivot, so that it pulls as the trigger goes forward, instead of rearward. That 2nd-2nd sear has an additional spring, and the main spring is obviously much, much stronger.

It seems like one of those "should work in theory" deals, that are much harder to do in reality.